The present invention relates to a radio transceiver for providing duplex radio communications by alternately transmitting and receiving on a time division basis. This system is called time division duplex or TDD. In such a system, the antenna needs to be connected at any instant to either the transmitter or receiving but not to both simultaneously.
A transmitter and receiver operate in time division duplex to transmit and receive respectively in different time slots of a time division multiple access (TDMA) frame period. A known transmitter/receiver or transceiver is illustrated in FIG. 1. A transceiver 10 includes a transmitter section 12 and a receiver section 14. The transmitter section 12 and receiver section 14 are connected to a common antenna 16 by means of a transmit/receive switch 22. The transmit/receive switch 22 alternately connects the antenna 16 to either the transmitter section 12 for transmission or the receiver section 14 for reception of signals. The transmit/receive switch 22 is controlled by a control means in a known manner which is not illustrated. For transmission, the transmit/receive switch 22 connects the antenna 16 to the transmitter section 12, wherein the signals to be transmitted are amplified in a power amplifier 16 and low pass filtered in a filter 20 which removes unwanted harmonics introduced by the power amplifier 18. The filtered signals are then applied to the antenna and transmitted. For reception, the transmit/receive switch 22 connects the antenna 16 to the receiver section 14 wherein signals that are received by the antenna 16 are sent to the receiver section and are filtered by the receiver filter 24 and are then further processed accordingly to a known manner. The transmit/receive switch 22 is usually a diode or MOSFET device. Certain kinds of communications systems must meet transmit spectral component (or side band) internodulation requirements. These include all those which have variable power modulation envelopes such as DAMPS (USA Digital Advanced Mobile Phone Service), PDC (Japan Personal Digital Cellular phone) and other common digital telephone protocols and all other forms of AM and QAM (quadrature phase related amplitude modulation). Transmit receive switches, particularly those made of diodes and FET switches, can distort the modulation envelope because their conduction is affected by the transmitted signal. This will require increased size, additional biasing or other undesirable and costly methods to combat the problem.
A further problem of using a simple T/R switch antenna coupling is that power may be reflected back from the antenna to the transmitter which effectively changes the power amplifier load impedance which can degrade the amplifier's own intermodulation characteristics. It is undesirable to combat this effect by increasing the power handling capability of the amplifier, especially in portable equipment, as this reduces power efficiency and thus battery life. High power amplifiers may damage T/R switch components if there is a large amount of reflected power from the antenna.
Another way to form a transceiver is to use a multiport circulator or isolator as illustrated in FIG. 2. In FIG. 2, a three port circulator 36 is used to connect a transmit section 32 and a receive section 34 with an antenna 38. FIG. 2 illustrates the functioning of the circulator 36. A B C illustrate connections and in this example, it can be assumed that the circulator has a throughput attenuation of approximately 0.8 dB from the connection A to B, from B to C, and from C to A. The reverse isolation is at least 12 dB, i.e., from connection B to A, and A to C, and finally from C to B. For proper functioning of the circulator, all inputs and outputs are loaded with an impedance of 50 ohms whereby the back isolation can be kept high. Thus, the circulator allows for signals to be transmitted to pass from the transmitter to the antenna with low attenuation but not to the receiver, (if all transmit power is absorbed by the antenna). In the prior art, using this circulator antenna duplex connection means any transmit power reflected by the antenna would be conducted to the receiver which must therefore have increased power handling capability for this eventuality. If the transmitter and receiver operated in somewhat different frequency bands, then the receiver input filter 40 would in turn reflect the transmitter power once again to the transmitter, producing the undesirable effects described above. This may happen to a lesser degree even if the frequency is the same due to receiver filter mismatches, which are common. However, in the prior art, signals received at another transmitter's frequency pass to the receiver where they are reflected by the receiver input filter 40 since the signals are out-of-band signals. Signals reflected from the receiver and back to the circulator will, in the prior art, be passed to the transmitter with low attenuation by the circulator where the reflected signal may appear as an impedance mismatch if the reflection is of the transmitter's own signal or would give rise to so-called back intermodulation if the reflected signal is another transmitter's signal.
One way to prevent the reflected signal from being passed to the transmitter is to use a four port circulator as illustrated in FIG. 3. In FIG. 3, a transmitter section 52 is connected to a first port of a circulator 54, an antenna 60 is connected to a second port of the circulator 54, and a receive section 56 is connected to a third port of the circulator 54. Finally, a load 58 is connected to the fourth port of the circulator 54 wherein signals reflected by the receive section 56 are diverted to the load 58 rather than the transmitter section 52. The four port circulator will isolate the transmitter from any antenna reflections but not the receiver. It forms a satisfactory antenna coupling apparatus only if there is no antenna reflection but otherwise reflected transmitter power flows into the receiver where it may overload it. The power reflected from the receiver will go to the load and therefore not effect the transmitter. It should be noted, however, that two non-reciprocal 180 degree paths are required for a four port device as compared with one for a three port device. Since this phase shifter, usually ferrite, forms the bulk of the circulator, a four port device is twice as big and more than twice as costly as a three port device.
Thus, there is a need for a method and an apparatus for preventing reflected signals from being transmitted back to a transmit section of a transceiver without introducing unwanted losses within the system.